In the Mahabharata, Karna learns the use of weapons from the sage Parashurama. To be accepted, he says he is a Brahmin. “It was for this that I introduced myself to thee as a person of thy own race,” he admits in the Ganguli translation.
When Parashurama discovers the deception, he curses him. Karna will forget the secret of the Brahmastra, the most powerful weapon, at the critical moment when he needs it against his enemy.